<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    #btn:hover{
      background-color: red;
    }
    .wrap{
      width: 100%;
      height: 100vh;
      overflow: hidden;
      position: fixed;
      top: 0;
      left: 0;
      background-color: rgba(0, 0, 0, .5);
    }
  </style>
</head>
<body>
  <p>hello</p>

  <h2>world</h2>

  <button id="btn">click me</button>

  <div class="wrap"></div>
</body>
</html>